The earliest developmentally regulated events that mark a gene for lineage-specific expression is still poorly understood. However, these early epigenetic and DNA occupancy events are key for regulating gene expression patterns that subsequently regulate developmental processes. The B cell developmental system is an excellent and tractable system to explore epigenetic regulatory mechanisms, as extensive analysis has led to the phenotypic and functional characterization of specific developmental stages that can be readily identified and isolated by cell surface markers. The immunoglobulin (Ig) genes are very well characterized genes that are differentially controlled during B cell development, and that utilize large scale, global regulatory mechanisms (somatic rearrangement and Ig locus contraction), as well as more localized regulatory mechanisms (enhancer activation, inducible transcription). Therefore, they are outstanding model genes to explore the molecular events regulating gene expression. We will utilize the Ig system to determine the early epigenetic events that initially target the Ig kappa locus during B cell development. We will utilize embryonic stem (ES) cell as well as complementary in vivo and ex vivo approaches to define the earliest epigenetic events that lead to large-scale Ig locus contraction, transcriptional activation, and Ig gene rearrangement during B cell development. Early events in B cell development are initiated by, and are critically dependent upon, the transcription factor PU.1. We developed a PU.1-null ES cell system expressing various PU.1 mutants that will enable us, in conjunction with conditional PU.1 knockout systems, to determine the PU.1- dependent functions important for changes in chromatin structure at the Ig kappa locus (Aim 2) and for B cell development (Aim 3). Finally, despite utilizing the same recombination machinery, the IgH and IgL loci are differentially accessible to the recombination machinery during early B cell development. Our preliminary results suggest that an important mechanism for controlling inaccessibility of the Ig kappa locus at the pro-B cell stage is binding of transcription factor STAT5 to a site that overlaps the central PU.1 binding site in the Ig kappa 3' enhancer as well as to sites flanking the intron enhancer. We will assess the consequences of STAT5 binding on enhancer activity, kappa locus transcription, and somatic rearrangement and will determine if competitive displacement represents a novel mechanism for developmental regulation of Ig gene function (Aim 4). We anticipate our studies will specifically elucidate novel regulatory mechanisms at the Ig: locus that control its developmental expression, thereby regulating the consequent progression of B cell development. On a more global level, we predict that these studies will reveal new paradigms for developmental control mechanisms mediated through a single enhancer regulatory element, thereby providing insights into the developmental disorders and/or malignancies caused by aberrant expression of lineage-specific or developmentally restricted genes.